The present invention relates to recording disk systems and more particularly, the invention relates to the control of positioning a transducer head precisely above any of the recording tracks of the disk in a disk drive.
Disk systems of the type to which the invention pertains are usually constructed with means for spinning a disk, e.g. a so-called "floppy" disk, about its axis. A transducer (or a pair of transducers) is mounted on a suitable mount constructed as a slide carriage or the like which is positioned by a solenoid, commonly referred to in this art as a voice coil motor. The motor is under specific control for positioning the transducer. This control involves usually two modes, operated under different conditions. In one mode, called a seek operation, the motor moves the transducer and carriage from one track to another one. This control operation may be carried out to cause the carriage to move as fast as possible, accelerating over a portion of the distance to be traversed and decelerating for the remainder. In a simplified but slower version one may include a constant speed phase. The second or positioning mode servo controls the position of the transducer above the center of the track on which it records or which it reads.
For purposes of these controls, a position sensor is provided, e.g. in the form of an optical grating representing the track positions. A detector on the carriage scans these gratings and provides a signal representative thereof. This signal is basically oscillatory in nature (though the signal contour is usually triangular rather than sinusoidal). The number of excursions or pulses sensed in this manner by the grating scanner and detector during movement of the carriage represents the number of tracks being passes across by the transducer. These pulses are often called detent pulses. The number of detent pulses is made to agree with a number externally commanded to the controller for causing the transducer to change tracks. The latter number is sometimes referred to in disk technology as cylinder difference, and the command pulses are called cylinder pulses. The detent pulses and the cylinder pulses are the inputs for the speed control of the carriage to change its track positions. The signal contour of the detector output is used directly to servo the voice coil motor so that the transducer is and remains above the selected track. Within a limited range that position signal has directly the contour of an error signal.
The servo and speed control as described is well developed, and satisfactorily working circuits are well known and incorporated in disk drives on the market. This is particularly true with regard to large disk file units, using hard surface disks, usually assembled in a horizontally oriented stack with vertical axis, and the voice coil motor and carriage axis are oriented both horizontally and on or parallel to a radial line of the disk or disks. As far as the smaller, "floppy" disk types units are concerned, I discovered the following problem.
Usually, the disk in such a unit is vertically oriented, but the voice coil motor and carriage still move on a horizontal axis. That, however, is not always the case. For some reason or another, that axis may have a vertical component, i.e. the unit is tilted. The axis may even be oriented straight up or down. It follows that the resulting force acting on the movable coil in the motor and being combined with and connected to the carriage, differs with that orientation, all other parameters remaining the same. Thus, if the control circuit for the two modes above is designed and rated to energize the motor in a particular manner, the different situations represented by different position and speed inputs will produce a definite behavior and response of the coil carriage combination only for a particular axis orientation. If the axis is differently oriented, a different gravity component will be superimposed modifying the control conditions for the carriage. Consequently, the servo conditions and particularly the accuracy and tolerances in the positioning of the transducer differ, they depend on the amount of tilt. Of course, one could trim adjust each such unit and calibrate it on the basis of the expected tilt. However, that tilt may be unforeseen at times and even vary. Moreover, such individual calibration is quite cumbersome. Recalibrating for every tilt change is outright impractical and even impossible.